site of several limestone caves near Burgos in northern Spain, known for the abundant human (genus Homo) remains discovered there beginning in 1976. The site called Sima del Elefante (“Pit of the Elephant”) contains the earliest evidence of humans in western Europe—fragments of a jawbone and teeth date to 1.1–1.2 million years ago. The nearby site of Gran Dolina contains human remains dating to about 800,000 years ago and some of the earliest tools found in western Europe. Paleoanthropologists who first described the fossils attributed them to a new species, H. antecessor, which they proposed as the ancestor of modern humans (H. sapiens) owing to certain distinctly modern facial features. Other researchers, however, hesitate to accept this assertion and group the fossils with similar remains classified as H. heidelbergensis. One of the most astonishing discoveries at Atapuerca is a cave called Sima de los Huesos (“Pit of the Bones”), where more than 1,600 human fossils, including...